chore: refactor handle_function_call() into smaller functions (#965)

Overall, `codex.rs` is still far too large, but at least there's less
indenting now that things have been moved into smaller functions.

This will also make it easier to introduce the `local_shell` tool in
https://github.com/openai/codex/pull/961.

---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/965).
* #961
* __->__ #965

codex-rs/core/src/codex.rs

@@ -40,6 +40,7 @@ use crate::config::Config;
 use crate::conversation_history::ConversationHistory;
 use crate::error::CodexErr;
 use crate::error::Result as CodexResult;
+use crate::error::SandboxErr;
 use crate::exec::ExecParams;
 use crate::exec::ExecToolCallOutput;
 use crate::exec::SandboxType;
@@ -1042,26 +1043,75 @@ async fn handle_function_call(
 ) -> ResponseInputItem {
     match name.as_str() {
         "container.exec" | "shell" => {
-            // parse command
-            let params: ExecParams = match serde_json::from_str::<ShellToolCallParams>(&arguments) {
-                Ok(shell_tool_call_params) => ExecParams {
-                    command: shell_tool_call_params.command,
-                    cwd: sess.resolve_path(shell_tool_call_params.workdir.clone()),
-                    timeout_ms: shell_tool_call_params.timeout_ms,
+            let params = match parse_container_exec_arguments(arguments, sess, &call_id) {
+                Ok(params) => params,
+                Err(output) => {
+                    return output;
+                }
+            };
+            handle_container_exec_with_params(params, sess, sub_id, call_id).await
+        }
+        _ => {
+            match try_parse_fully_qualified_tool_name(&name) {
+                Some((server, tool_name)) => {
+                    // TODO(mbolin): Determine appropriate timeout for tool call.
+                    let timeout = None;
+                    handle_mcp_tool_call(
+                        sess, &sub_id, call_id, server, tool_name, arguments, timeout,
+                    )
+                    .await
+                }
+                None => {
+                    // Unknown function: reply with structured failure so the model can adapt.
+                    ResponseInputItem::FunctionCallOutput {
+                        call_id,
+                        output: crate::models::FunctionCallOutputPayload {
+                            content: format!("unsupported call: {}", name),
+                            success: None,
                         },
+                    }
+                }
+            }
+        }
+    }
+}
+
+fn to_exec_params(params: ShellToolCallParams, sess: &Session) -> ExecParams {
+    ExecParams {
+        command: params.command,
+        cwd: sess.resolve_path(params.workdir.clone()),
+        timeout_ms: params.timeout_ms,
+    }
+}
+
+fn parse_container_exec_arguments(
+    arguments: String,
+    sess: &Session,
+    call_id: &str,
+) -> Result<ExecParams, ResponseInputItem> {
+    // parse command
+    match serde_json::from_str::<ShellToolCallParams>(&arguments) {
+        Ok(shell_tool_call_params) => Ok(to_exec_params(shell_tool_call_params, sess)),
         Err(e) => {
             // allow model to re-sample
             let output = ResponseInputItem::FunctionCallOutput {
-                        call_id,
+                call_id: call_id.to_string(),
                 output: crate::models::FunctionCallOutputPayload {
                     content: format!("failed to parse function arguments: {e}"),
                     success: None,
                 },
             };
-                    return output;
+            Err(output)
+        }
+    }
 }
-            };
 
+async fn handle_container_exec_with_params(
+    params: ExecParams,
+    sess: &Session,
+    sub_id: String,
+    call_id: String,
+) -> ResponseInputItem {
     // check if this was a patch, and apply it if so
     match maybe_parse_apply_patch_verified(&params.command, &params.cwd) {
         MaybeApplyPatchVerified::Body(changes) => {
@@ -1176,14 +1226,37 @@ async fn handle_function_call(
                 },
             }
         }
-                Err(CodexErr::Sandbox(e)) => {
+        Err(CodexErr::Sandbox(error)) => {
+            handle_sanbox_error(error, sandbox_type, params, sess, sub_id, call_id).await
+        }
+        Err(e) => {
+            // Handle non-sandbox errors
+            ResponseInputItem::FunctionCallOutput {
+                call_id,
+                output: FunctionCallOutputPayload {
+                    content: format!("execution error: {e}"),
+                    success: None,
+                },
+            }
+        }
+    }
+}
+
+async fn handle_sanbox_error(
+    error: SandboxErr,
+    sandbox_type: SandboxType,
+    params: ExecParams,
+    sess: &Session,
+    sub_id: String,
+    call_id: String,
+) -> ResponseInputItem {
     // Early out if the user never wants to be asked for approval; just return to the model immediately
     if sess.approval_policy == AskForApproval::Never {
         return ResponseInputItem::FunctionCallOutput {
             call_id,
             output: FunctionCallOutputPayload {
                 content: format!(
-                                    "failed in sandbox {:?} with execution error: {e}",
+                    "failed in sandbox {:?} with execution error: {error}",
                     sandbox_type
                 ),
                 success: Some(false),
@@ -1192,7 +1265,7 @@ async fn handle_function_call(
     }
 
     // Ask the user to retry without sandbox
-                    sess.notify_background_event(&sub_id, format!("Execution failed: {e}"))
+    sess.notify_background_event(&sub_id, format!("Execution failed: {error}"))
         .await;
 
     let rx_approve = sess
@@ -1212,10 +1285,7 @@ async fn handle_function_call(
             // TODO(ragona): Isn't this a bug? It always saves the command in an | fork?
             sess.add_approved_command(params.command.clone());
             // Inform UI we are retrying without sandbox.
-                            sess.notify_background_event(
-                                &sub_id,
-                                "retrying command without sandbox",
-                            )
+            sess.notify_background_event(&sub_id, "retrying command without sandbox")
                 .await;
 
             // Emit a fresh Begin event so progress bars reset.
@@ -1290,42 +1360,6 @@ async fn handle_function_call(
         }
     }
 }
-                Err(e) => {
-                    // Handle non-sandbox errors
-                    ResponseInputItem::FunctionCallOutput {
-                        call_id,
-                        output: FunctionCallOutputPayload {
-                            content: format!("execution error: {e}"),
-                            success: None,
-                        },
-                    }
-                }
-            }
-        }
-        _ => {
-            match try_parse_fully_qualified_tool_name(&name) {
-                Some((server, tool_name)) => {
-                    // TODO(mbolin): Determine appropriate timeout for tool call.
-                    let timeout = None;
-                    handle_mcp_tool_call(
-                        sess, &sub_id, call_id, server, tool_name, arguments, timeout,
-                    )
-                    .await
-                }
-                None => {
-                    // Unknown function: reply with structured failure so the model can adapt.
-                    ResponseInputItem::FunctionCallOutput {
-                        call_id,
-                        output: crate::models::FunctionCallOutputPayload {
-                            content: format!("unsupported call: {}", name),
-                            success: None,
-                        },
-                    }
-                }
-            }
-        }
-    }
-}
 
 async fn apply_patch(
     sess: &Session,